The present invention concerns a method for pressing a paper web or equivalent and for dewatering the web, in which a hot-pressing stage is utilized where the wet paper web is pressed in direct contact with a cylinder face or a corresponding roll face that has been heated to a temperature higher than 100.degree. C.
The present invention further concerns apparatus intended for carrying out this method, such apparatus comprising a hot cylinder or an equivalent roll which has a smooth heated mantle face which can be heated to a temperature higher than 100.degree. C. before reaching direct contact with the web to be pressed.
The most common prior art mode of dewatering fibrous webs, in particular paper and board webs, is to pass the web through a press nip formed by two rolls situated opposite one another. As is well-known, one or two press fabrics are used in the dewatering nips, which carry the water removed from the web further and act as a fabric carrying the web forwardly.
With increasing production rates of paper machines, the dewatering performed as nip pressing has become a bottle neck that limits increasing of the running speeds. This is due to the fact that the press nips formed by a pair of rolls have a short area, so that with high speeds the residence time of the web in these press nips remains short. However, a certain time is required in order to remove the water from the web into the hollow face of a roll or into the press fabric, especially due to the flow resistance of fiber structure of the web.
In a manner known in the prior art, several press nips placed one after the other have been used, either so-called compact press sections, an example of which is the Valmet "Sym-Press" .TM. press section, or several separate press nips situated one after the other. Nip presses, however, require relatively large space, especially if separate press nips situated one after the other are used. On the other hand, a compact construction of press sections causes difficulties in the optimal positioning of the different components during replacement of press rolls and press fabrics, as well as, e.g. in the disposal of paper broke during operation.
In nip presses, suction rolls are commonly used which are relatively expensive components and which consume suction energy. In suction rolls, a perforated mantle must be used, which causes problems in the mechanical strength of the suction rolls.
If attempts are made to increase the dewatering capacity in nip presses by increasing the nip pressure, the limit is reached with a certain linear load at which an increased nip pressure is no longer helpful, since the structure of the web no longer endures the compression.
Attempts have been made to extend the compression area in roll nips by using rolls of larger diameter and soft press fabrics, but even with these efforts the limits of economically feasible embodiments are soon reached.
Due to the problems described above and due to other reasons, so-called extended-nip presses have been developed in recent years. In this respect, reference is made by way of example to U.S. Pat. Nos. 3,783,097; 3,808,092; 3,808,096; 3,840,429; 3,970,515; 4,201,624; and 4,229,253, as well as to the Valmet Finnish Pat. Nos. 65,104; 70,952; and 71,369.
It has been known to use steam boxes or equivalent heating devices in connection with the press section in the prior art, by which the temperature of the water contained in the web to be pressed and of the fiber structure is raised so as to alter the viscosity of the water and the elastic properties of the web in such a manner that the dewatering is intensified. By means of these heating devices, the dry solids content of the web after the press section can be increased by only a few percentage points.
So-called hot-pressing methods are also known in the prior art, in which respect reference is made by way of example to U.S. Pat. No. 4,324,613, according to which the paper web is pressed in a roll nip in which one of the rolls or cylinders has been heated by means of surface heating to a temperature higher than 100.degree. C. In this nip, the surface water in the paper web can be vaporized and the pressurized vapor blows water which has been pressed into the intermediate spaces in the fiber structure into the paper, into the press felt. The dry solids content achieved by means of this prior art hot-pressing method is quite good, but there is a problem of the short nip time in a high-speed machine, because the compression time in a roll nip is only about 1 to 3 ms, so that the vaporization does not have enough time to be started properly unless the roll temperature is extremely high (on the order of 500.degree. C.). The high temperature of the roll results in problems, in particular with respect to the strength of the press fabric and of the roll.
With respect to the prior-art hot-pressing methods, reference is further made to the paper The Institute of Paper Chemistry, "Impulse Drying". In the method described in this paper "Impulse Drying", attempts have been made to eliminate the problem of the noted U.S. Pat. No. 4,324,613, i.e. The short nip time, so that instead of a roll nip, an extended nip is used which is formed by a heated roll or cylinder and a so-called extended-nip shoe. Thus, considerably more time is allowed for the vaporization of the water in the surface of the paper web as compared with the roll nip construction of the noted U.S. Pat. No. 4,324,613. However, a problem that remains is the high compression pressure (60-120 bar) that is required, in particular with thin paper qualities. This high compression pressure causes problems of lubrication of the glide shoe and the glide belt in the extended nip, such problems being further increased by the high temperature.
With respect to the prior art related to the hot-pressing technique, reference is further made to the Finnish Patent Application No. 853273 (corresponding to the International Pat. Appl. PCT/SE No. 85/00009, priority SE No. 84 00256-7, Jan. 19, 1984). In the method suggested in this cited publication, the paper web is pressed in a roll nip so that the press fabric is heated from outside the nip by means of heating devices. In the nip, water is compressed out of the paper web in the direction of this heated fabric. The allegedly good dry solids content is probably achieved, among other factors, in that a layer of vapor is thereby formed between the hot press fabric and the paper to be pressed, said vapor layer allegedly efficiently preventing rewetting of the paper. Problems of the method include, among other factors, both the production of a heat-resistant press fabric and the short nip time. On the whole, this method does not appear convincing and operable, at least not in its present stage of development.
Prior art related to the present invention further includes the so-called normal hot-pressing, which was preliminarily mentioned above and which is carried by using, e.g., a steam box for additional heating of the paper web. This mode of pressing is very common, e.g. in the Valmet Sym-Press II .TM. press section. However, in this method the temperature of the paper web always remains below 100.degree. C. so that no "blowing-through" of pressurized vapor or a corresponding pressing result is produced in the nip.
A "displacement pressing" method is also known in the prior art, in which pressurized air or steam is pressed through the paper web during the pressing stage, and water which has been pressed into the fiber structure can be removed from the fiber structure of the paper web. This method does not belong to the hot-pressing methods per se. Suggestions of equipment suitable for production machines for this method have not been made. A difficulty is arranging the blowing-through in the pressing zone.